Image processing apparatus

ABSTRACT

A system for performing gain correction of the image of an object obtained by image capture. A first white image is obtained by image capture at a first time with no object present, and a second white image is obtained by image capture at a second time later than the first image, with no object present. The first and second white images are then compared to determine if they meet predetermined criteria. If the predetermined criteria is met, a third white image is provided based on the comparison of the first and second white images, and the image of the object obtained by image capture is gain corrected based on the third white image.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image processing apparatus,an imaging device, an image processing system, an image processingmethod and a computer-readable storage medium storing program code forthe image processing method, for performing a gain correction process tomake uniform the gain of pixels forming an image which is obtainedthrough radiation imaging using an image pickup device composed of aplurality of pixels.

[0003] 2. Description of the Related Art

[0004] In radiation imaging, an object is irradiated with radiation, andthe intensity distribution of radiation transmitted through the objectis detected.

[0005] Specifically, the following method for radiation imaging iswidely used.

[0006] A combination of a “fluorescent screen” (or a “intensifyingscreen”), which emits fluorescence in response to radiation, and asilver film is set up with an object. The fluorescent screen and silverfilm and the object are then irradiated. The fluorescent screen convertsradiation into visible light and a latent image of the object is formedon the sliver film. The silver film having the latent image of theobject is then subjected to a chemical process. The silver film thenpresents a visible image (a radiation image of the object).

[0007] A radiation image thus obtained is an analog photograph, and isused for diagnostic imaging and examination.

[0008] Computed radiographic apparatuses (hereinafter referred to as“CR” apparatuses) using an imaging plate (hereinafter referred to as“IP”) with photostimulable phosphor applied thereon are now in use.

[0009] The CR apparatus emits photostimulated luminescence when the IPprimarily excited by the irradiation of radiation is subjected to asecondary excitation by visible light such as a red laser. Thephotostimulated luminescence is detected by a photosensor such as aphotomultiplier. Image data (radiation image data) thus obtained is usedto output a visible image on a photosensitive material or a cathode raytube.

[0010] The above-mentioned CR apparatus, which is a digital imagingapparatus, may be called an indirect digital imaging apparatus becauseit requires an imaging process of reading in response to a secondaryexcitation.

[0011] The CR apparatus is an “indirect imaging apparatus” because acaptured image (through radiation imaging) is not instantly presented atthe moment the image is taken. This is also the case as with a techniquein which a radiation image is taken as an analog photograph.

[0012] There have been recently developed apparatuses which capture adigital radiation image using photoelectric conversion means (imagepickup devices such as charge-coupled devices) composed of a matrix ofpixels, each being a tiny photoelectric converter or switching element.

[0013] Radiation imaging apparatuses having a charge-coupled device ortwo-dimensional amorphous silicon image pickup device with phosphordeposited thereon are disclosed in, for example, U.S. Pat. No.5,418,377, U.S. Pat. No. 5,396,072, U.S. Pat. No. 5,381,014, U.S. Pat.No. 5,132,539, and U.S. Pat. No. 4,810,881.

[0014] Since these apparatuses instantly display a captured radiationimage, they are called a direct digital imaging apparatus.

[0015] The advantage of the direct or indirect digital imaging apparatusover the analog photographing technique is due to filmless operation, alarge amount of acquired information through image processing and easeof building a database.

[0016] The advantage of the direct digital imaging apparatus over theindirect digital imaging apparatus is due to immediacy. Because of itsimmediacy, a radiation image through radiation imaging is displayed onthe spot. In the clinic field typically in need of urgency, immediacy isimportant.

[0017] In the direct digital imaging apparatus using the charge-coupledimage pickup device, the gain of each pixel forming the image pickupdevice is uniform. To produce a uniform output from the image pickupdevice with respect to the input image, gain correction is required on apixel by pixel basis.

[0018] Image capturing for gain correction is called calibration. A usertypically performs calibration on a regular basis.

[0019] Specifically, variations in gain in pixels in the image pickupdevice change with time under the influence of operational conditions.To acquire a satisfactory output image, a proper calibration must beperformed in response to operational conditions of the image pickupdevice at startup.

[0020] During calibration, the imaging device irradiates an entireeffective imaging area with an object (a subject) removed. The imagethus obtained (hereinafter referred to as a “white image” or a “gainimage”) is stored. Thereafter, actual radiation imaging (clinicalimaging) is performed. Specifically, the object is set up and radiationimaging (clinical imaging) is performed. Gain variations of the imagethus obtained (a clinical image) are corrected using the prestored whiteimage.

[0021] However, if there is any fault in the white image resulting fromcalibration in the conventional digital imaging apparatus, that faultmigrates to all subsequently taken images until a subsequentcalibration.

[0022] For example, when the irradiation of an entire image field withthe radiation of the imaging apparatus is limited through a radiationdiaphragm aperture during calibration, a resulting white image isobtained from an actually irradiated area of the entire field. If gaincorrection is performed on the actually obtained image using that whiteimage, an incomplete gain correction is performed on an area which wasnot irradiated during calibration.

[0023] Besides the irradiation field limitation due to the radiationdiaphragm aperture, the white image can become faulty due to theinclusion of a foreign object.

[0024] If the white image used in the gain correction is faulty, thefault migrates to subsequent images to be gain-corrected, resulting inpoor gain-corrected images.

SUMMARY OF THE INVENTION

[0025] It is an object of the present invention to provide an imageprocessing apparatus which presents a white image appropriate for use inthe correction of an image of an object.

[0026] The present invention in one aspect relates to an imageprocessing apparatus and includes a correction unit for performing acorrection process on an image of an object, obtained through imagecapturing, with a second correction image obtained through imagecapturing, and a determination unit for determining the fitness of thesecond correction image for the correction process, wherein thedetermining unit determines the fitness of the second correction imagefor the correction process based on a result of a comparison of a firstcorrection image obtained through image capturing and the secondcorrection image.

[0027] The present invention in another aspect relates to an imageprocessing method and includes a step of performing a correction processon an image of an object, obtained through image capturing, with asecond correction image obtained through image capturing, and a step ofdetermining the fitness of the second correction image for thecorrection process, wherein the determining step determines the fitnessof the second correction image for the correction process based on aresult of a comparison of a first correction image obtained throughimage capturing and the second correction image.

[0028] The present invention in yet another aspect relates to aprocessing software program for performing the function of an imageprocessing apparatus performing a correction process on an image of anobject, obtained through image capturing, with a second correction imageobtained through image capturing. The program includes program code fora step of determining the fitness of the second correction image for thecorrection process, wherein the determining step determines the fitnessof the second correction image for the correction process based on aresult of a comparison of a first correction image obtained throughimage capturing and the second correction image.

[0029] The above arrangements present a white image appropriate for usein the correction process of the image of the object.

[0030] Further objects, features and advantages of the present inventionwill be apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a block diagram showing the construction of a radiationimage apparatus of a first embodiment of the present invention;

[0032]FIG. 2 is a flow diagram illustrating a white image formationprocess in the radiation imaging apparatus;

[0033]FIG. 3 is a flow diagram illustrating a white image formationprocess in a second embodiment of the present invention;

[0034]FIG. 4 is a flow diagram illustrating a white image formationprocess in a third embodiment of the present invention;

[0035]FIG. 5 is a flow diagram illustrating a white image formationprocess in the third embodiment of the present invention;

[0036]FIG. 6 is a flow diagram illustrating a white image formationprocess in the third embodiment of the present invention;

[0037]FIGS. 7A and 7B show a display screen illustrating an inputrequest to a user in the third embodiment of the present invention;

[0038]FIG. 8 shows a display screen illustrating an input request to auser in the third embodiment of the present invention; and

[0039]FIG. 9 shows a display screen illustrating an input request to auser in the third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The embodiments of the present invention are now discussedreferring to the drawings.

[0041] First Embodiment

[0042] A first embodiment of the present invention is now discussed inconnection with a radiation imaging apparatus 100 shown in FIG. 1.

[0043] The radiation imaging apparatus 100 examines a white imageobtained during calibration, determining whether the white image is fitfor gain correction. A fault taking place in the white image is thusprevented from migrating to images to be gain-corrected.

[0044] <General Construction of the Radiation Imaging Apparatus 100>

[0045] Referring to FIG. 1, the radiation imaging apparatus 100 includesa radiation generator 103 for emitting a radiation (such as X rays) froma radiation emitting tube 101, an imaging button 102 for triggering theradiation generator 103 for the generation of radiation, an imaging unit106 for capturing a radiation image of an object 105 in response toradiation generated by the radiation emitting tube 101, an operationunit 116 for inputting a variety of settings for radiation imaging,three memories 111-113, a storage medium 114, and a CPU (CentralProcessing Unit) 109 for generally controlling the operation of theradiation imaging apparatus 100. These units are interconnected to eachother to exchange data through a bus 110.

[0046] The imaging unit 106 includes a radiation detector (a digitalradiation detector) 107 and an analog-to-digital converter 108.

[0047] The digital radiation detector 107 is placed at a locationexposed to the radiation from the radiation emitting tube 101, andincludes an image pickup device having a photodetector, fabricated ofamorphous silicon and TFTs (Thin-Film Transistors) and a phosphordeposited on the front thereof, and a driver and controller thereof,although these components are not shown.

[0048] The analog-to-digital converter 108 converts the output signal(the imaging signal) of the digital radiation detector 107 and providesa digital output signal to the bus 110.

[0049] The memory 111 (memory a) stores a variety of processing softwareprograms for the CPU 109 to generally control the operation of theradiation imaging apparatus 100. The memory 111 is used when the CPU 109performs a calculation process on the image.

[0050] The memory 112 (memory b) stores a digital image signal (digitalimage data) output by the analog-to-digital converter 108.

[0051] The memory 113 (memory c) stores a white image obtained duringcalibration.

[0052] The storage medium 114 stores gain-corrected image data, asopposed to the image data stored in the memory 112.

[0053] The operation unit 116, connected to the bus 110 via an interface(I/Fa) 115, includes control buttons for inputting a variety ofsettings, and has an image displaying function.

[0054] Through the operation unit 116, a user displays image data storedin the storage medium 114 using the image displaying function andperforms a variety of operations.

[0055] The radiation generator 103, connected to the bus 110 via aninterface (I/Fb) 104, generates the radiation (X rays) from theradiation emitting tube 101 in response to the pressing of the imagingbutton 102 or an instruction from the CPU 109.

[0056] Operational Sequence of the Radiation Imaging Apparatus 100

[0057] The operational sequence of the radiation imaging apparatus 100is now discussed.

[0058] A user (a radiological technologist) aligns an object (a subject)105 with the imaging unit 106, and presses the imaging button 102 tocommand the CPU 109 to start imaging.

[0059] In response to an imaging operation start command at the pressingof the imaging button 102 by the user, the CPU 109 controlsinitialization of the digital radiation detector 107, and then controlsthe radiation generator 103 to cause the radiation emitting tube 101 toemit radiation.

[0060] The digital radiation detector 107 is exposed to the radiationemitted from the radiation emitting tube 101, which is then transmittedthrough the subject 105. The digital radiation detector 107 is thusexposed to the distribution of the transmitted radiation that reflectsthe internal structure of the subject 105.

[0061] The digital radiation detector 107 two-dimensionally performsphotoelectric conversion in response to two-dimensional light intensitydistribution of the received radiation, thereby capturing an analogradiation image signal of the subject 105. The analog radiation imagesignal is then fed to the analog-to-digital converter 108.

[0062] The analog-to-digital converter 108 converts the analog imagesignal from the radiation detector 107, resulting in digital image data(radiation image data). The radiation image data is stored in the memory112.

[0063] The memory 113 then stores a gain-correcting white image, whichis obtained in a white image formation process to be discussed in detaillater.

[0064] The CPU 109 reads the white image stored in the memory 113, andperforms the gain correction process on the radiation image data storedin the memory 112 (a gain correction calculation process).

[0065] The CPU 109 stores the radiation image data subsequent to thegain correction process into the storage medium 114.

[0066] This completes the imaging operation.

[0067] Manipulating the operation unit 116, the user may read theradiation image data (the gain-corrected image data) from the storagemedium 114 to develop the digital image on film or to feed the digitalimage to a diagnostic imaging monitor, depending on the purpose ofapplication.

[0068] White Image Formation Process of the Radiation Imaging Apparatus100

[0069] To perform a correct gain correction process in the aboveoperational sequence, a proper white image must be first acquiredthrough calibration.

[0070] In the first embodiment, the CPU 109 executes a processingprogram in accordance with a flow diagram shown in FIG. 2 to acquire aproper white image for the gain correction process.

[0071] The white image formation process is performed when the user setsthe operation mode of the radiation imaging apparatus 100 to acalibration mode by operating the operation unit 116.

[0072] Step S201

[0073] When the radiation imaging apparatus 100 enters into acalibration mode, the CPU 109 queries the user whether to startcalibration. For example, the CPU 109 queries the user whether to startcalibration on the display function of the operation unit 116, andprompts the user to enter a reply (“Calibrate” or “No calibrate”).

[0074] When the reply from the user is “No calibrate”, the CPU 109performs step S213 to be discussed later. When the reply from the useris “Calibrate”, the CPU 109 performs the calibration process, startingwith step S202.

[0075] Step S202

[0076] When the user selects “Calibrate” in step S201, the CPU 109controls the digital radiation detector 107 to put the radiation imagingapparatus 100 in a calibration enable state in which imaging is possiblewith no subject 105 included.

[0077] After ensuring that the digital radiation detector 107 is readyfor calibration imaging (with the preparation for imaging completed),the CPU 109 displays a label to that effect using the display functionof the operation unit 116, and waits for a calibration imaging startcommand provided by the user.

[0078] When the user inputs the calibration imaging start command, theCPU 109 performs control as discussed in the section for the<Operational sequence of the radiation imaging apparatus 100>. However,the imaging operation is performed with no subject 105 included in theirradiation field of the apparatus 100. The radiation image thusobtained is a candidate as a white image to be used in the gaincorrection process.

[0079] The process to be discussed is to determine whether an obtainedwhite image is fit for use in the gain correction process, in otherwords, whether a calibration imaging has been properly performed.

[0080] To determine the fitness of the white image, the dose (amount ofradiation) and irradiation field of radiation, and the presence orabsence of a foreign object are checked. When all fitness criteria aresatisfied, the calibration is considered as being normally completed.

[0081] Step S203

[0082] The CPU 109 treats the white image as a candidate obtained instep S202 as “W”.

[0083] Step S204

[0084] The CPU 109 inspects the dose of radiation.

[0085] The inspection of the radiation dose is performed, because theeffective area of the digital radiation detector 107 (the photosensitivesurface of the imaging unit) must be entirely irradiated with radiationat an appropriate dose level with no object 105 being present. Further,the radiation dose needs to be inspected from the standpoint ofperforming proper gain correction and preventing noise from migrating tothe images.

[0086] Specifically, noise attached to the white image migrates to theradiation images taken thereafter each time the gain correction processis performed. For this reason, the white image must be captured underlow noise level conditions.

[0087] However, if the dose level is too low, noise from the digitalradiation detector 107 and the analog-to-digital converter 108 becomesrelatively high in level, affecting the white image. If the dose levelis too high, the digital radiation detector 107 and theanalog-to-digital converter 108 in the input/output characteristicsthereof suffer from poor linearity. The white image becomes invalid forthe gain correction process.

[0088] To determine whether the radiation dose is at an appropriatelevel, the CPU 109 extracts pixels in a central area (for example anarea having a matrix of 100 pixels by 100 pixels) of the digitalradiation detector 107 (the photosensitive surface of the imaging unit)less sensitive to the shading effect of the radiation in the white imageW in step S203, and calculates the mean of pixel values.

[0089] Step S205

[0090] The CPU 109 determines the mean pixel value acquired in step S204falls within a predetermined reference range.

[0091] When it is determined that the mean pixel value falls out of thepredetermined reference range (a proper dose-level irradiation is notperformed), the CPU 109 performs step S214 to be discussed later. Whenit is determined that the mean pixel value falls within thepredetermined range (a proper dose-level irradiation is performed), theCPU 109 continues at step S206.

[0092] Step S206

[0093] The CPU 109 performs an irradiation field test.

[0094] This test is needed because the white image, if obtained with theeffective area of the digital radiation detector 107 (the photosensitivesurface of the imaging unit) partly unirradiated during the calibration,cannot correctly perform a gain correction process in the pixels outsidethe irradiated area.

[0095] In step S206, the CPU 109 reads the white image Wo obtained in aprevious calibration from the memory 113, divides a white imagecandidate W by the white image Wo, and stores a resulting image W′(=W/Wo) in the memory 112.

[0096] The purpose of the division is to roughly correct gain variationsin the digital radiation detector 107 (gain variations across pixels ofthe image pickup device), and to increase examination accuracy. An imagefault examination is performed after removing gain variations in thedigital radiation detector 107. Step S206 is performed for the followingtwo reasons. 1) Gain variations in the digital radiation detector 107typically range from several percent to tens of percent; on the otherhand, variations to be examined in the examination to be discussed is onthe order of several percent. 2) Although the gain variations in thedigital radiation detector 107 slightly vary depending on imagingconditions and ambient temperature, the magnitude thereof is generallyconstant. For the reasons 1) and 2), the gain variations in the digitalradiation detector 107 are roughly corrected by dividing the currentwhite image by the previous white image, and the image fault examinationis performed to increase the imaging accuracy. In the first embodiment,the white image candidate W is divided by the white image Wo.Alternatively, a difference between the white image Wo and the whiteimage candidate W may be employed.

[0097] The reason the white image W is divided by the white image Woobtained in the previous calibration imaging is as follows. The usermust regularly calibrate the imaging system to maintain it in asatisfactory operating condition. The calibration is intended to correctchronological change in sensor gain variations and to avoid correctionfault due to a change in operational conditions. In the firstembodiment, a previous white image is used to determine a newcalibration for a fitness test. If the apparatus is used daily,yesterday's white image is used to test today's calibration, and today'swhite image will be used to test tomorrow's calibration. In a preferredembodiment, the latest (previous) white image Wo from among regularlyobtained white images Wo is used. The present invention is not limitedto this. The white image Wo may be updated regularly or at any time asthe user prefers, or on a non-regular basis.

[0098] Step S207

[0099] The CPU 109 subtracts pixels on both sides of, and immediatelyabove and below, a pixel W′ of interest stored in the memory 112 fromthe pixel W′ of interest to form a two-dimensional differential image.The CPU 109 binarizes the differential image, thereby determining anedge of change in the radiation dose.

[0100] Step S208

[0101] The CPU 109 determines from the detection result from step S207whether the irradiation field is appropriate.

[0102] When the CPU 109 determines that the irradiation field is notappropriate (i.e., the effective imaging area is not entirelyirradiated), the CPU 109 performs step S214 to be discussed later. Whenthe CPU 109 determines that the irradiation field is appropriate (i.e.,the effective imaging field is entirely irradiated), the CPU 109continues at step S209.

[0103] The determination in step S207, serving as a criterion in thedetermination of the irradiation field, is discussed by way example, andthe present invention is not limited this method.

[0104] Step S209

[0105] The CPU 109 inspects the presence or absence of a foreign object.

[0106] The calibration imaging can be performed with a foreign objectincluded. If the subject 105 is imaged with the foreign object removed,and if the radiation image is then subjected to the gain correctionprocess using the white image with the foreign object included, gain canbe excessively raised at the location of the foreign object. The foreignobject test is thus performed to avoid the inclusion of the foreignobject in the radiation image after the gain correction process. Theforeign object may be dust, remnants of imaging agents or handwritingwith a pencil core.

[0107] In step S209, the CPU 109 divides the image W′ stored in thememory 112 into tiny segments of 100 pixels by 100 pixels, and thendetermines the mean pixel values Eij and the standard deviation Sij ateach segment. Here, “ij” represent the coordinates of each segment.

[0108] Step S210

[0109] The CPU 109 determines from the mean pixel values Eij and thestandard deviations Sij obtained in step S209 whether a foreign objectis included.

[0110] Specifically, a mean value E of surrounding segments of a segmentij having a mean value of Eij (here eight segments) is determined byequation (1). $\begin{matrix}{E = {\left\{ {{\sum\limits_{x = {i - 1}}^{i + 1}{\sum\limits_{y = {j - 1}}^{j + 1}E_{xy}}} - E_{ij}} \right\}/8}} & (1)\end{matrix}$

[0111] The mean value E is compared with Eij. A mean value S of standarddeviations of surrounding segments of the segment ij having a standarddeviation Sij (here eight segments) is determined by equation (2).$\begin{matrix}{S = {\left\{ {{\sum\limits_{x = {i - 1}}^{i + 1}{\sum\limits_{y = {j - 1}}^{j + 1}S_{xy}}} - S_{ij}} \right\}/8}} & (2)\end{matrix}$

[0112] The mean value S of the standard deviations is compared with thestandard deviation Sij. These comparison results indicate whether themean value Eij and the standard deviation Sij vary, and thereforedetermines whether a foreign object is included. For example, the meanvalue Eij indicating a variation of 1% or more with respect to the meanvalue E determines that a foreign object is present. The standarddeviation Sij indicating a variation three times as large as, or largerthan, that in the mean value S of the standard deviations determinesthat a foreign object is present.

[0113] The determination criteria (a variation of 1% or more and threetimes as large as or larger than) are dependent on the S/N ratio of theradiation imaging apparatus 100.

[0114] When it is determined that a foreign object is present, the CPU109 performs step S214. When it is determined that no foreign object ispresent, the CPU 109 continues at step S211.

[0115] Step S211

[0116] When determination results in steps S205, S208, and S210 are allnormal (passed the tests), the CPU 109 updates the white image stored inthe memory 113 with the white image (W) obtained in this calibration.

[0117] Specifically, the CPU 109 divides the white image W by the meanpixel value (Wmean) of the white image W for normalization, therebyacquiring a white image Wo (=W/Wmean). The white image Wo is then storedin the memory 113.

[0118] Step S212

[0119] The CPU 109 displays through the display function of theoperation unit 116 a message indicating that the calibration has beensuccessfully completed.

[0120] Step S213

[0121] Subsequent to step S212 or when the user selects “No calibrate”in the above-mentioned step S201, the CPU 109 displays through thedisplay function of the operation unit 116 a message indicating thatcalibration has ended.

[0122] The radiation imaging apparatus 100 now exits the calibrationmode, and is ready for normal imaging (for the subject 105).

[0123] Step S214

[0124] When any one of determination results in steps S205, S208 andS210 is abnormal, the imaging may have been performed at an improperdose level, the effective imaging area may not have been entirelyirradiated or a foreign object may have been included. The CPU 109displays the abnormal test result through the display function of theoperation unit 116.

[0125] The CPU 109 then starts over at step S201, and the user is againasked whether or not to perform the calibration.

[0126] Through the white image formation process of the firstembodiment, a proper white image is always acquired for the gaincorrection process. Thereafter, the proper white image is then used toperform the gain correction on the radiation image obtained throughradiation imaging (for the subject 105). A gain-corrected image is thusreliably obtained.

[0127] In the first embodiment, initial values for a white image to bestored in the memory 113 are acquired in a dedicated mode for acquiringthe white image initial values at the installation of the radiationimaging apparatus 100.

[0128] Specifically, when the radiation imaging apparatus 100 isinstalled, the operation mode of the radiation imaging apparatus 100 isset to the dedicated mode (a service mode) and is then operated. Thisimaging operation is different from the sequence shown in FIG. 2. Theradiation imaging apparatus 100 merely performs a calibration imaging,and stores a resulting radiation image in the memory 113 as a whiteimage. In this way, no error is introduced because the comparison (theprocess step S206 shown in FIG. 2) cannot be performed between the whiteimage resulting from the previous calibration imaging and the whiteimage resulting from this calibration imaging.

[0129] The tests are conducted to examine the dose level, theirradiation field, and the presence or absence of a foreign object inthe first embodiment. The test items are not limited to these three. Forexample, when the radiation imaging apparatus 100 uses a movable grid, acheck may be made to make sure that the grid correctly moves, and thatthe mesh of the grid is correctly imaged. Further, a check may be madeto make sure that any image acquisition hardware does not create datacontamination.

[0130] In the first embodiment, the CPU 109 carries out the processshown in FIG. 2 by executing the processing program (in software).Alternatively, the process may be carried out in hardware. The use ofthe hardware shortens process time.

[0131] Second Embodiment

[0132] In a second embodiment, the radiation imaging apparatus 100 shownin FIG. 1 performs a white image formation process in accordance with aflow diagram shown in FIG. 3.

[0133] The difference between the first embodiment (see FIG. 2) and thesecond embodiment in the white image formation process lies in that thesecond embodiment allows the user to perform calibration imaging aplurality of times.

[0134] Performing a plurality of calibrations is intended to removerandom noise superimposed on the white image.

[0135] The “random noise” refers to radiation quantum noise andapparatus generating noise.

[0136] The greater the number of calibration (the calibration count),the effect of noise is significantly canceled. This is attributed to thefact that the incident X rays are extremely small in number comparedwith the visible light rays. Several fluctuating X ray quanta reach thesensor, thereby causing random noise to be directly superimposed on aninput image. If a single calibration is made, and if a uniform imageresulting from the same dose level is gain-corrected by the white image,the noise level of X ray quantum noise is raised by the square root of2. To reduce the noise level, the calibration imaging count is increasedto smooth the noise.

[0137] A larger number of calibrations cancels the effect of noise more,while involving the larger number of operational steps for imaging,namely increasing work load to the user.

[0138] In the second embodiment, the calibration imaging count isselected by the user.

[0139] In the flow diagram shown in FIG. 3, steps identical to thosewith reference to FIG. 2 are designated with the same step number, andthe detailed discussion thereof is skipped.

[0140] Step S301

[0141] When the radiation imaging apparatus 100 is put into thecalibration mode, the CPU 109 requests the user to input the calibrationimaging count. For example, through the display function of theoperation unit 116, the CPU 109 displays a screen for receiving an inputabout the calibration count, and requests the user to input a desiredcalibration imaging count on the screen.

[0142] The user then inputs a desired calibration count on the screenprovided by the display function of the operation unit 116.

[0143] The CPU 109 sets the number of calibrations input by the user in“sum” while initializing a counter n, indicating successful imagingcycles, to “1”, and the white image W prior to normalization to “0”.

[0144] The CPU 109 stores the imaging count, sum, the count n, and thewhite image W in the memory 112.

[0145] Step S302

[0146] The CPU 109 queries the user whether to continue the calibrationimaging. For example, through the display function of the operation unit116, the CPU 109 queries the user whether to continue the calibrationimaging, thereby prompting a reply to the query (“Continue calibration”or “Exit calibration”).

[0147] When the user reply is “Exit calibration”, the CPU 109 performsstep S213. When the user reply is “Continue calibration”, the CPU 109continues at step 202.

[0148] Step S202-step S210

[0149] When the user reply is “Continue calibration”, the CPU 109performs control to put the radiation imaging apparatus 100 into acalibration imaging ready state. In response to an imaging start commandfrom the user, the CPU 109 initiates the imaging operation (step S202).The radiation image is now regarded as a white image candidate (Wn) atan n-th calibration (step S203).

[0150] The CPU 109 examines the dose level and the irradiation field ofthe radiation, and the presence or absence of a foreign object (stepsS204-S210).

[0151] Step S310

[0152] When determination results in steps S205, S208, and S210 are allnormal (passed the tests), the CPU 109 adds the white image Wn at then-th calibration to the white image W prior to normalization in thememory 112 (W=W+Wn).

[0153] Step S303

[0154] The CPU 109 displays, through the display function of theoperation unit 116, a message indicating that the n-th calibrationimaging has successfully been completed.

[0155] Step S304, and Step S305

[0156] The CPU 109 adds “1” to the count n of the counter n (step S304),and determines whether the updated count of the counter n exceeds thecalibration count sum.

[0157] When the determination result is not n>sum, the CPU 109 returnsto step S302 to start over. When the determination result is n>sum, theCPU 109 continues at step S211.

[0158] Step S211

[0159] When the determination result in step 305 is n>sum, in otherwords, when the calibration count desired by the user is completed, theCPU 109 divides the white image W stored in the memory 112 by a meanpixel value (Wmean) of the white image W for normalization, acquiringthe image Wo (=W/Wmean). The image Wo is stored in the memory 113 as awhite image to be used for the gain correction process.

[0160] Step S212

[0161] The CPU 109 displays, through the display function of theoperation unit 116, a message indicating that the sum of calibrationshas been successfully completed.

[0162] Step S213

[0163] Subsequent to step S212 or when the user selects “Exit calibrate”in the above-mentioned step S301, the CPU 109 displays through thedisplay function of the operation unit 116 a message indicating that thecalibration has ended.

[0164] The radiation imaging apparatus 100 now exits the calibrationmode, and is ready for normal imaging (for the subject 105).

[0165] The white image W stored in the memory 112 is not updated whenthe radiation imaging apparatus 100 exits as a result of an abortedcalibration.

[0166] Step S214

[0167] When any one of determination results in steps S205, S208, andS210 is abnormal, the CPU 109 displays the abnormal test result throughthe display function of the operation unit 116.

[0168] The CPU 109 starts over at step S201. The user is again askedwhether to perform the calibration. The count n of the counter is notupdated, and an n-th calibration is again performed.

[0169] In the white image formation process in the second embodiment,each of white images resulting from a plurality of calibrations isexamined in terms of fitness as a white image to be used for the gaincorrection process regardless of whether an imaging fault suddenly takesplace. For example, if ten white images are obtained from tencalibrations with one of the ten image at fault, the image fault isaveraged to one-tenth, but the image fault may migrate to nine normalimages. In accordance with the second embodiment, however, the fitnessdetermination is carried out for all white images one by one. Theappropriate white image is always used for the gain correction processfor the radiation imaging thereafter (for the subject 105). The properlygain-corrected image is thus reliably obtained. Since the calibrationcount is selected by the user himself, the user can adjust thecalibration count in a balanced manner in consideration of other jobs.

[0170] There are times when the effective imaging area of the imagingunit 106 is not entirely irradiated even with the aperture opened,because of a near range imaging. In this case, the calibration imagingneeds to be performed under the same conditions. When the irradiationfield for the white image candidate fails to cover the effective imagingarea, the radiation imaging apparatus 100 may leave the determination ofthe white image candidate to be valid/not valid to the user's selectionwhile the irradiation field test result is presented to the user, ratherthan immediately determining the white image candidate to not be validwith an abnormal message presented to the user as already discussed inconnection with the first and second embodiments.

[0171] When the irradiation field fails to cover the effective imagingarea during the calibration imaging, and when the irradiation fieldduring the calibration imaging and the irradiation field during clinicalimaging are not aligned, an artifact is introduced in the image afterthe gain correction process. But such a problem is resolved by the gaincorrection method disclosed in Japanese Laid-Open Patent No. 2-4545which is assigned to the same assignee of this invention.

[0172] Specifically, in this gain correction process, a white image isacquired beforehand from a calibration imaging with the entire effectiveimaging area fully irradiated. The obtained beforehand white image iscombined with a white image resulting from calibration imagingoperations performed with a predetermined period with the effectiveimaging area not entirely irradiated. Thus, a white image having theentire effective area irradiated results.

[0173] In another method, only the overlapping portion of a white imageresulting from calibration imaging and an image resulting from theimaging of an object may be output as an image.

[0174] The artifacts are eliminated or reduced through the gaincorrection methods above.

[0175] Third Embodiment

[0176] In a third embodiment, the radiation imaging apparatus 100performs a white image formation process in accordance with flowdiagrams shown in FIGS. 4, 5, and 6. FIG. 5 is a continuation of theflow diagram of FIG. 4, and both figures are linked by a mark *1. FIG. 6is a flow diagram that illustrates step S410 in more detail in FIG. 5,and both figures are linked by marks *2 and *3.

[0177] The difference between the white image formation process in thefirst embodiment (see FIG. 2) and the white image formation process inthe third embodiment lies in that the third embodiment allows the userto perform any plural number of calibrations. The difference between thewhite image formation process in the second embodiment (see FIG. 3) andthe white image formation process in the third embodiment lies in thatthe third embodiment allows the user to perform any plural number ofcalibrations with a different dose level. Further, the user is allowedto designate a determination area for a calibration quality test.

[0178] Performing a plurality of calibrations is intended to removerandom noise superimposed on the white image at each calibration.

[0179] The “random noise” refers to radiation quantum noise andapparatus generating noise.

[0180] A larger calibration count cancels the effect of noise more,while involving the larger number of operational steps for imaging. Inthe third embodiment, the calibration count is selected by the user.

[0181] The white images obtained from radiation of different dose levelsare intended to correct nonlinearity of the radiation detector. Even iflinearity is maintained in a middle region of a dose level, linearity ofoutput characteristics is destroyed by an error in an offset correctionin a low-dose level region. The linearity of the output characteristicsis also destroyed by saturated characteristics of the radiation detectorin a high-dose level region. The non-linearity is compensated for byselectively using white images having different dose-level regions inresponse to the output value.

[0182] The calibration quality determination area is set up by the user,just in case the effective imaging area is not entirely irradiated forsome reason. For example, this is the case when the imaging unit 106 andthe radiation emitting tube 101 are not sufficiently spaced apart.Another case is that an obstacle covers the imaging unit 106 forshielding for installation layout reasons or clinical concerns.

[0183] In any case, if radiation of a sufficient dose level fails toreach part of the effective imaging area during the calibration imaging,and a fitness test to be discussed later may determine the resultingwhite image to not be valid. To avoid such a determination, the userdesignates beforehand an invalid area, to exclude the designated invalidarea from the fitness test. The invalid result in the fitness test to bediscussed later lets the user know the cause of the insufficientirradiation of the invalid area.

[0184] Steps S401 through S419 in FIGS. 6, 7, and 8 are now discussed.

[0185] Step S401

[0186] When the radiation imaging apparatus 100 is put into thecalibration mode, the CPU 109 requests the user to input a calibrationeffective area.

[0187] For example, as represented by 7 a in FIG. 7A, the CPU 109displays an effective area 601 represented by a solid line and acalibration area represented by a broken line through the displayfunction of the operation unit 116. The user is thus requested to enterthe calibration imaging count on the screen. In the third embodiment,the input section of the operation unit 116 is a display having a touchsensor. Alternatively, a mouse may be used.

[0188] The calibration area 602 may be a previously used one at themoment the calibration mode is entered, i.e., with no area input, or maybe the entire area. When the user touches any two points within theeffective area, a rectangular calibration area 602 having a diagonalthat ends at the two designated points is displayed. The user can thusdesignate a desired calibration area on the screen presented by thedisplay function of the operation unit 116.

[0189] Step S402

[0190] The CPU 109 queries the user whether the calibration area 602formed in response to the user request is acceptable. For example, asrepresented by 7 b in FIG. 7B, the CPU 109 displays the calibration area602 designated in step S401. If the user is not satisfied for anyreasons including erroneous input, the user may press a NG (no good)button 603. The algorithm then returns to step S401.

[0191] When the user is satisfied with the shown calibration area 602,the user presses an OK button 604, and the CPU 109 continues at stepS403.

[0192] Steps S403-S406

[0193] The CPU 109 requests the user to enter the dose level ofradiation. The CPU 109 sets the dose level input by the user into“cal_no” while initializing the count N of the counter N, for countingsuccessful imaging cycles, to “1”. The CPU 109 stores the calibrationdose level cal_no and the count N in the memory 112 (step S403).

[0194] In succession, the CPU 109 requests the user to input thecalibration imaging count at each dose level. The CPU 109 sets theimaging count input by the user to “sum” while initializing the count nof the counter n, indicating successful imaging cycles, to “1”. The CPU109 stores the imaging count sum and the count n in the memory 112 (stepS404).

[0195] The input request and input at steps S403 and S404 are performedusing the display function and input function of the operation unit 116.

[0196] The CPU 109 displays, to the user, the calibration area selectedin step S402 and the calibration dose level and the imaging count ateach calibration dose level selected in steps S403 and S404, and queriesthe user whether the calibration plan is acceptable (step S405).

[0197] The execution of steps S403-S405 are performed using the displayfunction and input function of the operation unit 116 shown in FIG. 8.The calibration area 602 is shown inside the effective area 601. In thethird embodiment, the radiation dose is divided into three dose-levelregions 605. The user is allowed to select one from among the threelevels. In the third embodiment, a low dose level and a high dose levelcan be deselected, but a medium dose level must always be selected. Inother words, the minimum 1 (the medium dose level) to the maximum 3 (thelow dose level, the medium dose level, and the high dose level) areavailable. The number of steps in dose level may be increased. To inputthe dose level, the user selects a desired dose level, and presses acheck button on the left of each dose level label in a toggle fashion toselect or to deselect. With any check button already pressed at themoment of pressing an OK button 604, the calibration is performed underthe selected dose level.

[0198] In the third embodiment, the imaging count at each dose level isinput using a downward looking button for down counting and an upwardlooking button for upward counting while referring to the figure at animaging count 606. Any dose not selected at the dose level display 605presents zero with the downward looking button and the upward lookingbutton inoperative. A dose selected at the dose level display 605presents a number equal to or greater than 1. The imaging count settingat the moment the OK button 604 is pressed becomes the calibrationimaging count at each dose level.

[0199] The CPU 109 queries the user whether the displayed calibrationplan is acceptable. The display function and input function of theoperation unit 116 shown in FIG. 9 are used. If the user is notsatisfied for any reason including erroneous input, the user may pressthe NG button 603. The algorithm then returns to step S403. When theuser is satisfied with the calibration plan, the user presses the OKbutton 604, and the CPU 109 continues at step S407 (step S406).

[0200] Step S407

[0201] The CPU 109 initializes a plurality of white images W′₍₁₎, W′₍₂₎,W′₍₃₎, . . . , W′_((cal) _(—) _(no)) prior to normalization to “0”.

[0202] Step S408

[0203] The CPU 109 queries the user whether to continue the calibration.For example, the CPU 109 queries the user whether to start calibrationon the display function of the operation unit 116, and prompts the userto enter a reply (“Continue calibration” or “Exit calibration”).

[0204] When the reply from the user is “Exit calibration”, the CPU 109performs step S420 to be discussed later. When the reply from the useris “Continue calibration”, the CPU 109 performs the calibration process,starting with step S409.

[0205] Step S409

[0206] When the user selects “Calibrate” in step S408, the CPU 109controls the digital radiation detector 107 to place the radiationimaging apparatus 100 in a calibration enable state in which imaging ispossible with no subject 105 included.

[0207] After ensuring that the digital radiation detector 107 is readyfor the calibration imaging (with the preparation for imagingcompleted), the CPU 109 displays a label to that effect using thedisplay function of the operation unit 116, and waits for a calibrationimaging start command provided by the user.

[0208] When the user inputs the calibration imaging start command, theCPU 109 performs control as discussed in the section for the Operationalsequence of the radiation imaging apparatus 100. However, the imagingoperation is performed with no subject 105 in place. The radiation imagethus obtained is a candidate W_(((N−1)*sum+n)) as a white image to beused in the gain process.

[0209] Step S410

[0210] The CPU 109 tests the white image candidate W_(((N−1)*sum+n)) interms of its fitness as a calibration image. FIG. 6 shows test stepsS501-S507. In FIG. 5, steps S501-S507 are collectively designated afitness examination in step S410.

[0211] Test items include a dose test (step S501), an irradiation fieldtest (step S503), a foreign object test (step S504), a grid test (stepS505), a line noise test (step S506), and an abnormal bit test (stepS507). These test items, except for the dose test (step S501), needs thecomparison of the white image candidate W_(((N−1)*sum+n)) with aprevious white image W_((N)) performed in step S502. Although the dosetest must be performed first for this reason, the order of the remainingtest items is discretionary.

[0212] Step S501

[0213] The CPU 109 performs the dose test.

[0214] The checking of the radiation dose is performed, because theeffective area of the digital radiation detector 107 (the photosensitivesurface of the imaging unit) must be entirely irradiated with radiationat an appropriate dose level with the subject 105 not in place. Further,the radiation dose needs to be inspected from the standpoint ofperforming proper gain correction and preventing noise from migrating tothe images.

[0215] Specifically, noise attached to the white image migrates to theradiation images taken thereafter each time the gain correction processis performed. The white image must be captured under low noise levelconditions.

[0216] However, if the dose level is too low, noise from the digitalradiation detector 107 and the analog-to-digital converter 108 becomesrelatively high in level, affecting the white image. If the dose levelis too high, the digital radiation detector 107 and theanalog-to-digital converter 108 in the input/output characteristicsthereof suffer from poor linearity. The white image then becomes invalidfor the gain correction process.

[0217] To determine whether the radiation dose is at an appropriatelevel, the CPU 109 extracts pixels in a central area (for example anarea having a matrix of 100 pixels by 100 pixels) of the digitalradiation detector 107 (the photosensitive surface of the imaging unit)less sensitive to the shading effect of the radiation in the white imagecandidate W_(((N−1)*sum+n)) obtained in step S409, and calculates themean of pixel values.

[0218] The CPU 109 determines the mean pixel value acquired in step S409falls within a predetermined reference range.

[0219] When it is determined that the mean pixel value falls out of thepredetermined reference range (a proper dose-level irradiation is notperformed), the CPU 109 determines that the dose level has failed thedose test. When it is determined that the mean pixel value falls withinthe predetermined range (a proper dose-level irradiation is performed),the CPU 109 determines that the dose level has passed the dose test.

[0220] Step S502

[0221] The CPU 109 reads the white image W_((N)) obtained in a previouscalibration from the memory 113, divides a white image candidateW_(((N−1)*sum+n)) by the white image W_((N)), and stores a resultingimage W″ (=W_(((N−1)*sum+n))/W_((N))) in the memory 112.

[0222] The purpose of the division is to roughly correct gain variationsin the digital radiation detector 107 (gain variations across pixels ofthe image pickup device), and to increase examination accuracy. An imagefault examination is performed after removing gain variations in thedigital radiation detector 107. Step S502 is performed for the followingtwo reasons. 1) Gain variations in the digital radiation detector 107typically range from several percent to tens of percent; on the otherhand, variations to be examined in the examination to be discussed is onthe order of several percent. 2) Although the gain variations in thedigital radiation detector 107 vary slightly depending on imagingconditions and ambient temperature, the magnitude thereof is generallyconstant. For the reasons 1) and 2), the gain variations in the digitalradiation detector 107 are roughly corrected by dividing the currentwhite image candidate W_(((N−1)*sum+n)) by the previous white imageW_((N)), and the image fault examination is performed to increase theimaging accuracy. In the third embodiment, the white image candidateW_(((N−1)*sum+n)) is divided by the white image W_((N)). Alternatively,a difference between the white image W_((N)) and the white imagecandidate W_(((N−1)*sum+n)) may be employed.

[0223] The reason why the white image candidate W_(((N−1)*sum+n)) isdivided by the white image W_((N)) obtained in the previous calibrationimaging is as follows. The user must regularly calibrate the imagingsystem to keep it in a satisfactory operating condition. The calibrationis intended to correct chronological change in sensor gain variationsand to avoid correction fault due to a change in the operationalconditions. In the third embodiment, a previous white image is used todetermine a new calibration for a fitness test. If the apparatus is useddaily, a yesterday's white image is used to test the today'scalibration, and a today's white image will be used to test a tomorrow'scalibration. In a preferred embodiment, the latest (previous) whiteimage W_((N)) from among regularly obtained white images W_((N)) isused. The present invention is not limited to this. The white imageW_((N)) may be updated regularly or at any time at the user prefers, oron a non-regular basis.

[0224] Step S503

[0225] The CPU 109 performs the irradiation field test. The irradiationfield test is performed on the area set in step S401, with the remainingarea excluded as a test area.

[0226] The white image, which results during the calibration when theimaging area of the digital radiation detector 107 (the photosensitivesurface of the image pickup device) is not entirely irradiated, cannotperform a correct gain correction process on the pixels outside theirradiated area thereof.

[0227] The CPU 109 subtracts pixels on both sides of, and immediatelyabove and below, a pixel W″ of interest stored in the memory 112 fromthe pixel W″ of interest to form a two-dimensional differential image.The CPU 109 binarizes the differential image, thereby determining anedge of change in radiation dose.

[0228] The CPU 109 determines from the edge detection result whether theirradiation field is appropriate. When the CPU 109 determines that theirradiation field is not appropriate (i.e., the calibration area is notentirely irradiated), the CPU 109 determines that the irradiation fieldhas failed to pass the irradiation field test. When the CPU 109determines that the irradiation field is appropriate (i.e., thecalibration area is entirely irradiated), the CPU 109 determines thatthe irradiation field has passed the irradiation field test. The methodfor testing the irradiation field is discussed by way of example only,and the present invention is not limited to this method.

[0229] Step S504

[0230] The CPU 109 inspects the presence or absence of a foreign object.

[0231] The calibration imaging can be performed with a foreign objectincluded. If the subject 105 is imaged with that foreign object removed,and if the radiation image is then subjected to the gain correctionprocess using the white image with the foreign object included, gain canbe excessively raised at the location of the foreign object. The foreignobject test is thus performed to avoid the inclusion of the foreignobject in the radiation image after the gain correction process. Theforeign object may be dust, remnants of imaging agents, or handwritingwith a pencil core. The area to be tested is the area set in step S401,and the remaining area outside the set area is excluded from the testarea.

[0232] In step S504, the CPU 109 divides the image W″ stored in thememory 112 into tiny segments of 100 pixels by 100 pixels, and thendetermines the mean pixel values Eij and the standard deviation Sij foreach segment. Here, “ij” represent the coordinates of each segment.

[0233] The CPU 109 determines from the obtained mean pixel values Eijand the obtained standard deviations Sij whether a foreign object isincluded.

[0234] Specifically, a mean value E of surrounding segments of a segmentij having a mean value of Eij (here eight segments) is determined byequation (1). $\begin{matrix}{E = {\left\{ {{\sum\limits_{x = {i - 1}}^{i + 1}{\sum\limits_{y = {j - 1}}^{j + 1}E_{xy}}} - E_{ij}} \right\}/8}} & (1)\end{matrix}$

[0235] The mean value E is compared with Eij. A mean value S of standarddeviations of surrounding segments of a segment ij having a standarddeviation Sij (here eight segments) is determined by equation (2).$\begin{matrix}{S = {\left\{ {{\sum\limits_{x = {i - 1}}^{i + 1}{\sum\limits_{y = {j - 1}}^{j + 1}S_{xy}}} - S_{ij}} \right\}/8}} & (2)\end{matrix}$

[0236] The mean value S of the standard deviations is compared with thestandard deviation Sij. These comparison results indicate whether themean value Eij and the standard deviation Sij vary, and thereforedetermines whether a foreign object is included. For example, the meanvalue Eij indicating a variation of 1% or more with respect to the meanvalue E determines that a foreign object is present. The standarddeviation Sij indicating a variation of three times as large as orlarger than that in the mean value S of the standard deviationsdetermines that a foreign object is present.

[0237] The determination criteria (a variation of 1% or more and threetimes as large as or larger than) are dependent on the S/N ratio of theradiation imaging apparatus 100.

[0238] When it is determined that a foreign object is present, the CPU109 determines that the white image has failed to pass the foreignobject test. When it is determined that no foreign object is present,the CPU 109 determines that the white image has passed the foreignobject test.

[0239] Step S505

[0240] The CPU 109 performs a grid test.

[0241] The inclusion of the mesh of the grid in imaging occurs when gridmotion control is not correctly performed or when synchronization withirradiation timing is not correctly established. Since the frequency ofthe grid is known, the CPU 109 applies a Fourier transform on the imageW″ stored in the memory 112 in a direction perpendicular to the grid,and examines the frequency spectrum corresponding to the grid. Thus, thegrid test is easily performed. The area to be tested is the area set instep S401, and the remaining area outside the set area is excluded fromthe test area.

[0242] The CPU 109 determines whether the spectrum value of thefrequency region of the grid exceeds a predetermined threshold. Forexample, the predetermined threshold is set to be twice as high as anoise spectrum level.

[0243] When it is determined that the spectrum value of the frequencyregion of the grid exceeds the predetermined threshold, the CPU 109determines that the grid has failed to pass the grid test. When it isdetermined that the spectrum value of the frequency region of the gridis not higher than the predetermined threshold, the CPU 109 determinesthat the grid has passed the grid test.

[0244] Step S506

[0245] The CPU 109 performs the line noise test.

[0246] External noise, if added to a driver power supply circuit, causesnoise streaks aligned in the same direction to appear in an image in theimage pickup device used in the digital radiation detector 107, althoughthe level of the noise depends on the driving method of the image pickupdevice. This is because data transfer is performed concurrentlyhorizontally or vertically on a column by column basis. The externalnoise refers to electrical noise coming from an alternating currentpower supply or a ground line, or electromagnetic radiation noise. Thearea to be tested is the area set in step S401, and the remaining areaoutside the set area is excluded from the test area.

[0247] The CPU 109 performs the line noise test, by dividing a certainarea of the image W″ stored in the memory 112 into segments, andexamines the direction of generation of a noise streak on a segment bysegment basis, and the projection data of the noise streak with respectto a vertical direction. The CPU 109 compares the standard deviation ofthe projected data with a predetermined permissible value for theapparatus. The segmentation of the certain area of the image W″ isperformed to detect localized noise streaks or to avoid examinationaccuracy drop attributed to entire shading of the image. The aboveexamination may be performed on an entire area of interest. A frequencyanalysis using a Fourier transform may be performed.

[0248] When the standard deviation of the projection data exceeds thepermissible value of the apparatus, the CPU 109 determines the linenoise has failed to pass the line noise test. When the standarddeviation of the projection data is equal to or less than thepermissible value of the apparatus, the CPU 109 determines that the linenoise has passed the line noise test.

[0249] Step S507

[0250] The CPU 109 performs the abnormal bit test.

[0251] An abnormal bit occurs when a pixel defect takes place in thedigital radiation detector 107 or when a digital data transfer error iscreated. When the entire system operates normally, the image W″ becomesa substantially uniform image. By examining a difference between theoutput of each pixel and the mean value of the outputs of all pixels,the generation of an abnormal bit is easily detected. The area to betested is the area set in step S401, and the remaining area outside theset area is excluded from the test area.

[0252] The CPU 109 divides a certain area of the image W″ stored in thememory 112 into segments, and calculates the mean and the standarddeviation of the pixels at each segment. The CPU 109 determines whetherthe difference between the value of each pixel in each segment and themean value exceeds five times the standard deviation. If the criterionof five times is lowered, the detection rate of an abnormal pixel riseswhile the possibility of erroneously detecting a normal pixel as anerratic pixel increases. Conversely, if the criterion of five times israised, the possibility of erroneously detecting a normal pixel as anerratic pixel is lowered while the detection rate of an abnormal bitdrops. On the assumption that an erroneous detection occurs with acertain probability, a determination criterion may be defined by thenumber of detected abnormal bits to the number of all pixels.

[0253] When abnormality is found, the CPU 109 determines that theabnormal bit test is a failure. When abnormality is not found, the CPU109 determines that the abnormal bit test is a success.

[0254] Step S411

[0255] The CPU 109 references each of the results from steps S501, S503,S504, S505, S506, and S507, and then determines the following step. Ifthe test results from steps S501, S503, S504, S505, S506, and S507 provesuccessful, the CPU 109 performs step S412. When at least one of thesteps S501, S503, S504, S505, S506, and S507 is not successful, thealgorithm branches to step S421.

[0256] Step S412

[0257] The CPU 109 adds the calibration white image W_(((N−1)sum+n)) tothe white image W_((N)) prior to normalization stored in the memory 112and substitutes the result for the white image W_((N))(W_((N))=W_((N))+W_(((N−1)*sum+n))).

[0258] Step S413

[0259] The CPU 109 displays, through the display function of theoperation unit 116, a message indicating that the n-th calibration withthe N-th dose level has been normally completed.

[0260] Step S414 and Step S415

[0261] The CPU 109 adds “1” to the counter n (step S414), and determineswhether the updated count exceeds the imaging count sum designated bythe user.

[0262] When the determination result is not n>sum, the CPU 109 returnsto step S408 to start over. When the determination result is n>sum, theCPU 109 continues at step S416 (step S415).

[0263] Step S416

[0264] When the determination result in step 415 is n>sum, in otherwords, when the calibration count desired by the user is completed, theCPU 109 divides the white image W′_((N)) stored in the memory 112 by amean pixel value (Wmean) of the white image W′_((N)) for normalization,acquiring the image W_((N)) (=W′_((N))/Wmean). The image W_((N)) isstored in the memory 113 as a white image to be used for the gaincorrection process.

[0265] Steps S417 and S418

[0266] The CPU 109 adds “1” to the counter N (step S417), and determineswhether the count N exceeds the imaging count sum designated by theuser.

[0267] When it is not N>cal_no, the CPU 109 returns to step S408 tostart over. When it is N>cal_no, the CPU 109 continues at step S419(step S418).

[0268] Step S419

[0269] The CPU 109 displays, through the display function of theoperation unit 116, a message indicating that the calibration plandisplayed in step S405 has been successfully completed.

[0270] Step S420

[0271] Subsequent to step S419 or when the user selects “Exitcalibration” in the above-mentioned step S408, the CPU 109 displaysthrough the display function of the operation unit 116 a messageindicating that calibration has ended.

[0272] The radiation imaging apparatus 100 now exits the calibrationmode, and is ready for normal imaging (for the subject 105).

[0273] The white image W_((N)) stored in the memory 112 is not updatedwhen the radiation imaging apparatus 100 exits as a result of an abortedcalibration.

[0274] Step S421

[0275] When any one of determination results in steps S501, S503, S504,S505, S506, and S507 is abnormal, the CPU 109 displays the abnormal testresult through the display function of the operation unit 116.

[0276] For example, as shown in FIG. 9, the CPU 109 displays a messageindicating that calibration has failed. A test result display 607 listswhich test has failed and which test has succeeded. To let the user knowthe progress of calibration until the test failure, the screen displaysthe dose level at the failure of the dose level test at the dose leveldisplay 605 and the imaging count at the time of failure at the imagingcount display 606.

[0277] The CPU 109 starts over with step S408. In this way, the user isasked whether or not to start the calibration again.

[0278] The count n of the counter is not updated, and an n-thcalibration is again performed.

[0279] Using the white images W₍₁₎, W₍₂₎, . . . , W_((N)) createdthrough the above-referenced calibration process, the gain correction isperformed in the clinical imaging. In the white image formation processin the above embodiments, each of white images resulting from aplurality of calibrations is examined in terms of fitness as a whiteimage to be used for the gain correction process regardless of whetheran imaging fault suddenly takes place. For example, if ten white imagesare obtained from ten calibrations with one of the ten images at fault,the image fault is averaged to one-tenth, but the image fault maymigrate to the normal nine images. In accordance with the aboveembodiments, however, the fitness determination is carried out for allwhite images one by one. The appropriate white image is always used forthe gain correction process for the radiation imaging thereafter (forthe subject 105). The properly gain-corrected image is thus reliablyobtained. Since the calibration count is selected by the user himself,the user can adjust the calibration count in a balanced manner inconsideration of other jobs.

[0280] A storage medium storing program codes of the software programfor performing the functions of a host computer or mobile terminal ofthe first, second and third embodiments is supplied to a system or anapparatus, and the computer (CPU or MPU) of the system or the apparatusreads the program codes to carry out the program.

[0281] The program code read from the storage medium performs thefunction of the first and second embodiments. The storage medium storingthe program codes falls within the scope of the present invention.

[0282] Available as storage media for providing the program code are aROM, a floppy disk, a hard disk, an optical disk, a magnetooptical disk,a CD-ROM, a CD-R, a magnetic tape, a non-volatile memory card, and thelike.

[0283] The computer executes the supplied software program, therebyperforming the functions of the first, second and third embodiments.Furthermore, the program codes perform the functions of the first,second, and third embodiments in cooperation with the OS (OperatingSystem) running on the computer. Such program codes fall within thescope of the present invention.

[0284] The supplied program codes are written on a function expansionboard inserted into the computer or a memory provided on an functionexpansion unit connected to the computer. A CPU on the expansion boardor function expansion unit partly or entirely performs the process. Thefunctions of the first, second, and third embodiments are thusperformed. Such a system also falls within the scope of the presentinvention.

[0285] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. An image processing apparatus comprising: acorrection means for performing a correction process on an image of anobject, obtained through image capturing, with a second correction imagealso obtained through image capturing; and a determination means fordetermining a fitness of the second correction image for the correctionprocess, wherein the determining means determines the fitness of thesecond correction image for the correction process based on a result ofa comparison of a first correction image obtained through imagecapturing and the second correction image.
 2. An image processingapparatus according to claim 1 , wherein the determining meansdetermines the fitness of the second correction image for the correctionprocess based on a comparison result which is obtained by comparing thefirst correction image with each of a plurality of second correctionimages resulting from a plurality of image capturing actions.
 3. Animage processing apparatus according to claim 2 , wherein the imagecapturing by the image processing apparatus is radiation imaging, andthe plurality of image capturing actions are each performed with adifferent radiation dose.
 4. An image processing apparatus according toclaim 1 , further comprising a storage circuit for storing an imagecaptured through image capturing, wherein the determining means storesthe second correction image as the first correction image in the storagecircuit when the determining means determines that the second correctionimage is fit for the correction process.
 5. An image processingapparatus according to claim 1 , further comprising a notifying meansfor notifying of the determination result provided by the determiningmeans.
 6. An image processing apparatus according to claim 5 , whereinthe notifying means comprises a warning means for displaying a warninglabel when the determining means determines that the second correctionimage is unfit for the correction process.
 7. An image processingapparatus according to claim 5 , wherein the warning label presents theresults of fitness determinations to the correction process,determination item by determination item.
 8. An image processingapparatus according to claim 1 , wherein the comparison is performed bycarrying out a calculation on the second correction image with respectto the first correction image.
 9. An image processing apparatusaccording to claim 1 , wherein the comparison is performed by dividingthe second correction image by the first correction image.
 10. An imageprocessing apparatus according to claim 1 , wherein the comparison isperformed by determining a difference between the second correctionimage and the first correction image.
 11. An image processing apparatusaccording to claim 1 , wherein the determining means determines thefitness of the second correction image for the correction process, basedon information of a mean and a standard deviation of pixel values ineach of a plurality of segments, into which the second correction imageis divided.
 12. An image processing apparatus according to claim 1 ,wherein the image capturing is radiation imaging.
 13. An imageprocessing apparatus according to claim 1 , wherein the captured imageof the object is an image signal obtained in response to imagecapturing.
 14. An image processing apparatus according to claim 1 ,wherein the determining means determines the fitness of the secondcorrection image for the correction process, in terms of a foreignobject based on the result of the comparison between the firstcorrection image and the second correction image.
 15. An imageprocessing apparatus according to claim 1 , wherein the image capturingof the image processing apparatus is radiation imaging, and wherein thedetermining means determines the fitness of the second correction imagefor the correction process in terms of an irradiation area of radiationbased on the result of the comparison between the first correction imageand the second correction image.
 16. An image processing apparatusaccording to claim 1 , wherein the image capturing of the imageprocessing apparatus is radiation imaging, and wherein the determiningmeans captures the second correction image subsequent to an adjustmentof a dose of radiation.
 17. An imaging device for capturing an image ofan object with a ray transmitted through the object using an imagingmeans having a plurality of pixels, the imaging device comprising: acorrection means for performing a correction process on an image of anobject, obtained through image capturing, with a second correction imagealso obtained through image capturing; and a determination means fordetermining a fitness of the second correction image for the correctionprocess, wherein the determining means determines the fitness of thesecond correction image for the correction process based on a result ofa comparison of a first correction image obtained through imagecapturing and the second correction image.
 18. An image processingsystem comprising a plurality of apparatuses which are adapted tomutually communicate, with at least one of the apparatuses comprising: acorrection means for performing a correction process on an image of anobject, obtained through image capturing, with a second correction imagealso obtained through image capturing; and a determination means fordetermining a fitness of the second correction image for the correctionprocess, wherein the determining means determines the fitness of thesecond correction image for the correction process based on a result ofa comparison of a first correction image obtained through imagecapturing and the second correction image.
 19. An image processingmethod for an image processing apparatus, comprising: a step ofperforming a correction process on an image of an object, obtainedthrough image capturing, with a second correction image also obtainedthrough image capturing; and a step of determining a fitness of thesecond correction image for the correction process, wherein thedetermining step determines the fitness of the second correction imagefor the correction process based on a result of a comparison of a firstcorrection image obtained through image capturing and the secondcorrection image.
 20. An image processing method according to claim 19 ,wherein the determining step determines the fitness of the secondcorrection image for the correction process based on a comparison resultwhich is obtained by comparing the first correction image with each of aplurality of second correction images resulting from a plurality ofimage capturing actions.
 21. An image processing method according toclaim 20 , wherein the image capturing is radiation imaging, and theplurality of image capturing actions are each performed with a differentradiation dose.
 22. An image processing method according to claim 19 ,further comprising a step of storing an image captured through imagecapturing, wherein the determining step stores the second correctionimage as the first correction image in the storing step when thedetermining step determines that the second correction image is fit forthe correction process.
 23. An image processing method according toclaim 19 , further comprising a step of notifying of the determinationresult provided in the determining step.
 24. An image processing methodaccording to claim 23 , wherein the notifying step comprises adisplaying step of displaying a warning label when the determining stepdetermines that the second correction image is unfit for the correctionprocess.
 25. An image processing method according to claim 23 , whereinthe displaying step presents the results of fitness determinations tothe correction process, determination item by determination item.
 26. Animage processing method according to claim 19 , wherein the comparisonis performed by carrying out a calculation on the second correctionimage with respect to the first correction image.
 27. An imageprocessing method according to claim 19 , wherein the comparison isperformed by dividing the second correction image by the firstcorrection image.
 28. An image processing method according to claim 19 ,wherein the comparison is performed by determining a difference betweenthe second correction image and the first correction image.
 29. An imageprocessing method according to claim 19 , wherein the determining stepdetermines the fitness of the second correction image for the correctionprocess, based on information of a mean and a standard deviation ofpixel values in each of a plurality of segments, into which the secondcorrection image is divided.
 30. An image processing method according toclaim 19 , wherein the image capturing is radiation imaging.
 31. Animage processing method according to claim 19 , wherein the capturedimage of the object is an image signal obtained in response to imagecapturing.
 32. An image processing method according to claim 19 ,wherein the determining step determines the fitness of the secondcorrection image for the correction process, in terms of a foreignobject based on the result of the comparison between the firstcorrection image and the second correction image.
 33. An imageprocessing method according to claim 19 , wherein the image capturing ofthe image processing apparatus is radiation imaging, and wherein thedetermining step determines the fitness of the second correction imagefor the correction process in terms of an irradiation area of radiationbased on the result of the comparison between the first correction imageand the second correction image.
 34. An image processing methodaccording to claim 19 , wherein the image capturing of the imageprocessing method is radiation imaging, and wherein the determining stepcaptures the second correction image subsequent to an adjustment of doseof radiation.
 35. A processing software program for performing afunction of an image processing apparatus performing a correctionprocess on an image of an object, obtained through image capturing, witha second correction image also obtained through image capturing, theprogram comprising program code for a step of determining a fitness ofthe second correction image for the correction process, wherein thedetermining step determines the fitness of the second correction imagefor the correction process based on a result of a comparison of a firstcorrection image obtained through image capturing and the secondcorrection image.
 36. A storage medium storing a processing softwareprogram for performing a function of an image processing apparatushaving a function of performing a correction process on an image of anobject, obtained through image capturing, with a second correction imagealso obtained through image capturing, the program comprising programcode for a step of determining a fitness of the second correction imagefor the correction process, wherein the determining step determines thefitness of the second correction image for the correction process basedon a result of a comparison of a first correction image obtained throughimage capturing and the second correction image.
 37. An image processingmethod comprising: obtaining by image capture with no object present afirst white image at a first time; obtaining by image capture with noobject present a second white image at a second time which is later thanthe first time; comparing the first white image and the second whiteimage to determine if a predetermined criteria is met; providing a thirdwhite image based on the comparison of the first white image and thesecond white image, if the predetermined criteria is met; obtaining byimage capture an image of an object; and gain correcting the image ofthe object based on the third white image.